Electromagnetic interference (EMI) is a long-standing problem in the electronics industry. Such interference can arise as an unwanted byproduct of normal circuit operation or can be induced into a circuit by external sources, e.g., electrostatic discharge. In any event, the result of such interference is a noise signal which interferes with the desired operation of electrical circuitry.
While it is possible to reduce the effects of electromagnetic radiation by judicious circuit design, a thorough analysis of EMI in the design process is very complex and such analysis is often not done. Moreover, even if the analysis is done, EMI is virtually impossible to completely eliminate. In addition to the detrimental effects of EMI, many system applications place strict limits on the electromagnetic radiation emanating from electronic equipment. Indeed, FCC Part 15 requirements specifically pertain to such limits and cover both home and business environments. Consequently, electronic equipment manufacturers use numerous techniques to mitigate EMI, such as the use of capacitors, magnetic filters, toroids and shield structures on circuit boards or substrates. While such techniques can meet system performance objectives, there are situations where such techniques do not provide a satisfactory solution. For example, after a circuit has been designed and initially fabricated, EMI can exceed system objectives or FCC requirements. To meet such objectives or requirements using the prior art techniques requires the addition of components on or off the circuit pack and such additions often require a circuit board redesign. Or, in other situations, certain circuit boards meet system objectives while other similar circuit boards, due to an accumulation of component tolerances, do not. To overcome this problem, either some of the fabricated circuits have to be discarded or partially reassembled with components having tighter tolerances, or components with such tighter tolerances have to be utilized for all circuits, or additional EMI-suppressing components have to be utilized which are external to the circuit pack or substrate. None of these solutions is optimum from time and cost standpoint. In addition, the use of EMI-suppressing components which are external to the circuit pack or substrate is often undesirable in applications, e.g., customer premises equipment, where such external components are located in the wiring coupled to the equipment. These additions, aside from being unsightly, typically require the customer's consent and intervention. Moreover, FCC rules require that the external EMI-suppressing components be registered and supplied with the manufactured circuitry, and this requirement can be unsatisfactory because it limits the customer's ability to utilize one piece of circuitry with different sets of cables. It would, therefore, be advantageous if an EMI-reducing technique could be developed which would overcome all of the above-described limitations of the prior art.